It is often necessary to conduct an electrical charge or current from a rotating to a stationary member or vice versa. There also might be a further requirement that the charge or current be transported with the two sides being at unequal pressure. The conventional way to solve this problem is to use a combination of slip ring and a seal such as a ferrofluid or mechanical seal. The slip ring may use mercury between the rotating and stationary member or simply a low resistance spring-loaded mechanical contact such as gold alloy or silver graphite etc. against the rotating member.
Two industries where such devices are used are the semiconductor and computer industry. In the semiconductor industry, for example, the RF or DC sputtering performed under vacuum requires that the current be carried out from the atmosphere to the vacuum side, and from the stationary element (atmosphere side) to rotating (shaft) element (vacuum side). In the computer industry, the static charge built up at the disk in a rotating spindle needs to be grounded in addition to sealing hermetically the disk cavity for contamination-free operation.
The current ferrofluids used in ferrofluid seal apparatus are nonconducting with resistivity in the range of about 10.sup.10 ohm-cm or more. Thus when these fluids are used as ferrofluid sealants, they provide the ferrofluid sealing function, but do not conduct charge or current from the housing (stationary) to the rotating shaft or vice versa. Ferrofluids or magnetic colloids are liquids with magnetic properties in which ferromagnetic materials are colloidally suspended. These colloids are responsive to external magnetic fields. The response depends generally on the concentration and the type of ferromagnetic particles and also a function of shape and size of the particles.
A very stable magnetic colloid in a high magnetic field gradient requires very small ferromagnetic particles, typically less than 100 .ANG. diameter and coated with one or several layers of surfactants to prevent agglomeration in a particular liquid carrier. Ferrofluids are widely known and used and typical compositions are described, for example in U.S. Pat. No. 3,700,595, issued Oct. 24, 1972; U.S. Pat. No. 3,764,540, issued Oct. 9, 1973; and U.S. Pat. No. 4,430,239, issued Feb. 7, 1984. A particular process for preparing ferrofluid compositions are described in U.S. Pat. No. 3,917,538, issued Nov. 4, 1975; and U.S. Pat. No. 4,381,244, issued Apr. 26, 1983; U.S. Pat. No. 4,485,024, issued Nov. 27, 1984 and U.S. Pat. No. 4,356,098, issued Oct. 26, 1982 which describe various other techniques for synthesis of magnetic liquids.
In recent years such magnetic colloids or ferrofluids have been used for sealing in ferrofluid seals, as damping liquids in inertia dampers, as heat transfer liquids in the voice coil of loudspeakers, as bearing liquids and lubricants, and for domain detection, oil prospecting, and other applications.
The non-aqueous or oil based ferrofluids thus far produced exhibit very high resistivities, in the range of 10.sup.10 ohm-cm, and are basically considered dielectrics. Ferrofluids have been limited in applications where transport of electrical charge is required along with the sealing or other functions. It would be most desirable that besides sealing, damping and other properties, the magnetic colloids offer also improved electrical conductivity, particularly for use in computer and semiconductor seal application, such as computer disk drive; where the static charge accumulated at the disk can be grounded through a conductive ferrofluid and RF or DC Sputtering; where power can be fed from a stationary to a rotating member or vice versa.